Boarding Eurostar at a different station
You can board or alight at Ashford/Ebbsfleet providing the train stops there, even if your train is booked to St Pancras and vice versa. However please bear in mind that not all Eurostar trains are scheduled to stop at Ebbsfleet or Ashford so be sure to check ahead of time.
If your train does stop at Ebbsfleet or Ashford just check in for your journey at the station of your choice via the manual check-in.
For journeys from the Continent to the UK, you will only be able to board the train from the station indicated on your ticket.